The present invention relates to a current controller that is configured to control a current flowing through a solenoid of an electromagnetic proportional relief valve.
For example, a forklift is known as an industrial vehicle that includes an engine and a hydraulic pump driven by the engine. A forklift operates a hydraulic actuator with hydraulic oil discharged from a hydraulic pump. The forklift, for example, includes hydraulic cylinders that are hydraulic actuators configured to move the fork upward or downward and hydraulic cylinders that are hydraulic actuators configured to tilt the mast assembly. The forklift also includes a control valve that controls supply and drainage of hydraulic oil, which is supplied from the hydraulic pump, to and from the hydraulic cylinders. When the hydraulic pump is driven by the engine, the engine torque may become insufficient as the load on the hydraulic pump increases, which may cause the engine to stall. Thus, conventionally, configurations for avoiding such engine stalls have been proposed. For example, refer to Japanese Laid-Open Patent Publication No. 2015-187026.
The industrial vehicle described in the above publication employs an electromagnetic control valve to supply and drain hydraulic oil. When the hydraulic actuators are not being operated, the industrial vehicle is always determined to be in the off-load state. In the off-load state, the pressure in the hydraulic mechanism is released to the oil tank, so that the pressure in the hydraulic mechanism is low. If load is applied to the engine and the on-load period at that time is less than a predetermined time, the hydraulic mechanism is controlled to be in the on-load state to increase the pressure in the hydraulic mechanism. Thereafter, the industrial vehicle is returned to the off-load state to prevent an abrupt increase in the pressure, so that the engine is prevented from stalling.
Also, an engine may be prevented from stalling by using an electromagnetic proportional relief valve to maintain the pressure acting on the hydraulic pump (the pressure of the hydraulic oil) at a value less than or equal to a relief pressure. The relief pressure of the electromagnetic proportional relief valve is regulated by current flowing through the solenoid. The current flowing through the solenoid is controlled by a current controller in accordance with a current command calculated based on the current deviation, which is the difference between a target current value corresponding to the relief pressure and the current flowing through the solenoid (the actual current). The actual current is detected by a current detection circuit. Thus, if a wire is broken or instantaneous interruption occurs in the current detection circuit, the current flowing through the solenoid cannot be detected. In such a case, the current deviation is increased, and the current command is increased, accordingly.